1. Field of the Invention
The present invention relates to an antilock/traction modulator and more specifically to a brake control system for use in a vehicle which applies an appropriate brake force to vehicle wheels to prevent the wheels from slipping when the vehicle starts to move rapidly, and controls the brake force appropriately to prevent the vehicle wheels from skidding when the vehicle brakes.
2. Related Art
Recently, there has actively been developing an anti-lock/traction modulator having functions of an anti-lock control for preventing vehicle wheels from skidding when the vehicle brakes and a traction control for preventing the wheels from excessive slipping when the vehicle starts to move or accelerates rapidly, for a driver to drive a vehicle easily and improve abilities of start, acceleration and driving stability of the vehicle.
An example of the conventional anti-lock/traction modulator of this type is disclosed, for example, in U.S. Pat. Nos. 4,750,788 and 4,726,630.
FIG. 1 is a schematic view of a brake apparatus described in U.S. Pat. No. 4,750,788. A brake apparatus 100 is provided with a master cylinder 102, booster 103, hydraulic pump P, adjusting valve 106, change valves 104, 105 and 108-115, brake cylinders of driving and driven wheels, reservoir tank 107 and oil passages for selectively communicating these devices and valves.
The operation of the brake apparatus 100 is briefly described as follows.
(1) Normal Braking Operation
The condition of the change valves during the normal braking operation is as shown in FIG. 1. When a hydraulic pressure is generated in the master cylinder 102 by depressing a brake pedal, the hydraulic (brake) pressure is supplied to a brake cylinder for each of the vehicle wheels from outlet ports I, II of the cylinder 102 through the change valves 108, 111, 109 and 113.
(2) Anti-skid Control Operation
If the wheels skid during the braking operation, the change valves 104, 108 and 109 are changed from their condition shown in FIG. 1 and, at the same time, the hydraulic pump P is actuated. The brake pressure generated by the hydraulic pump P becomes equal in pressure to the hydraulic pressure generated by the master cylinder 102 owing to the operation of the adjusting valve 106 to which the hydraulic pressure from the master cylinder 102 is applied. Accordingly, the brake pressure generated by the hydraulic pump P becomes ready for being supplied to the wheel cylinders through tee valves 108 and 109. Under this condition, the valves 110 and 111, or 112 and 113 are controlled to open and close to control the brake force to hold, release and apply according to the condition of skid of each of the vehicle wheels, thereby achieving the anti-skid control.
(3) Traction Control Operation
During the traction control operation, the valves 104, 105 108, 109, 114 and 115 are changed from the condition shown in FIG. 1 and, at the same time, the hydraulic pump P is actuated. The brake pressure generated by the hydraulic pump P of an in-line type is supplied to the brake cylinders through valves 108 and 109 to thereby apply the brake force to the vehicle wheels. Under this condition, the valves 110 and 111 or 112 and 113 are controlled to open and close to control the brake force to hold, release and apply according to the condition of the slip of each of the driving wheels, thereby achieving the traction control.
The valves 104, 105 and 108-115 as well as the hydraulic pump P are controlled by a publicly known electronic control device (not shown) in accordance with output signals of wheel speed sensors for detecting the condition of the vehicle wheels (skidding or slipping).
As described above, the skid or slip of the vehicle wheels are appropriately controlled and, therefore, the vehicle can decelerate or stop stably and start to move smoothly.
However, the conventional anti-lock/traction modulator as described above has a disadvantage that if a part of the hydraulic pressure passage circuit for the traction control comes to fail or have a damage the passage circuit for the normal brake operation comes also to deteriorate which raises a serious problem in safety driving since the pressure passage circuit for the traction control communicates with that for the anti-skid control. Further, the TCS pump of an in-line type is required to pump up the brake fluid against the passage resistance generated between the master cylinder and the modulator, thereby lowering the ability of the pump.